• Preheat oven to 350°F. Prepare a 12-cup Bundt cake pan with a light coating of nonstick cooking spray.

Make The Cake Batter:

• In a medium bowl, whisk together the flour, cocoa, baking soda, and salt. Set aside.
  
• Place the chopped chocolate and melted butter in a microwave-safe bowl. Microwave at 50% power in 30-second increments, stirring between each, until fully melted and smooth. Allow to cool slightly.
  
• In a separate large bowl, whisk together the buttermilk, granulated sugar, eggs, and vanilla extract. Slowly whisk the melted chocolate mixture into the wet ingredients until combined.
  
• Gently fold the flour mixture into the wet ingredients until just combined. Do not overmix.

Prepare The Flan Mixture:

• In a blender or using an immersion blender, combine the sweetened condensed milk, whole milk, cream cheese, eggs, egg yolks, and vanilla extract. Blend for about 1 minute until the mixture is smooth and uniform.
  

Assemble And Bake The Cake:

• Pour the caramel sauce into the bottom of the pan.
  
• Carefully spread the chocolate cake batter over the caramel layer.
  
• Slowly pour the flan mixture over the cake batter—it will gently layer on top.
  
• Place the Bundt pan into a large roasting pan or deep baking dish. Carefully pour enough boiling water into the roasting pan to come halfway up the sides of the Bundt pan to create a water bath. This ensures even, gentle baking.
  
• Bake in the preheated oven for 75 to 90 minutes, or until a toothpick inserted into the cake portion (not the flan) comes out clean. Alternatively, use an instant-read thermometer to check for an internal temperature of 180°F.
  

Chill And Finish The Cake:

• Remove the Bundt pan from the water bath and transfer it to a wire rack. Allow the cake to cool to room temperature, about 1 hour. Then, refrigerate the cake in the pan for at least 8 hours or overnight to fully set the flan.
• To unmold the cake, dip the bottom of the Bundt pan in warm water for about 1 minute to loosen the caramel. Place a serving plate over the pan and carefully invert it. The cake should release easily, with the caramel sauce drizzling over the top and sides.
  
• Slice and serve cold.